Lubricating oils, including hydraulic oils and crankcase oils, often are used in environments in which the oil is subject to mechanical agitation in the presence of air. As a consequence, the air becomes entrained in the oil and also forms a foam.
Foam appears on the surface of an oil as air bubbles greater than 1 mm in diameter. Air entrainment refers to the dispersion within the oil of air bubbles less than 1 mm in diameter.
Air entrainment and foaming in lubricating compositions are undesirable phenomena. For example, air entrainment reduces the bulk modulus of the fluid resulting in spongy operation and poor control of a hydraulic system's response. It can result in reduced viscosity of a lubricating composition. Both air entrainment and foaming can contribute to fluid deterioration due to enhanced oil oxidation.
Air entrainment, however, is more problematic than foaming. Foaming is typically depressed in lubricating compositions by the use of antifoamant additives. These additives expedite the breakup of a foam, but they do not inhibit air entrainment. Indeed, some antifoamants, such as silicone oils typically used in diesel and automotive crankcase oils, are known to retard air release. The rate of air release and amount of air entrainment of lubricating compositions may be determined by the test method of ASTM D 3427. Indeed, the rate of air release referred to herein has been determined by that method.
U.S. Pat. No. 6,090,758 discloses that foaming in a lubricant comprising a slack wax isomerate is effectively reduced by use of an antifoamant exhibiting a spreading coefficient of about 2 mN/m without increasing the air release time. While the specified antifoamant does not degrade the air release time, further improvements in enhancing air release characteristics are desirable.
Many modern gasoline and diesel engines are designed to use the crankcase oil to function as a hydraulic fluid to operate fuel injectors, valve train controls and the like. For these functions, low air entrainment and rapid air release are indicative of high performance lubricants. Indeed, it is anticipated that in the future the rate of air release from engine lubricants will become more critical to the proper operation of internal combustion engines as engine designs evolve and become ever more complex.
U.S. Pat. No. 6,713,438 discloses a lubricating oil composition that exhibits improved air release characteristics. The composition comprises a basestock, typically a polyalphaolefin (PAO), and two polymers of different molecular weight. One of the polymers is a viscoelastic fluid having a shear stress greater than 11 kPa such as a high VI PAO, and the other preferably is a linear block copolymer.
The present invention provides desirable improvements in lubricant air release rates through the use of certain vinyl aromatic-olefin block copolymers.